Not Applicable
Not Applicable
1. Field of Invention
This invention relates to the field of gamma ray detection in a positron emission tomograph (PET) imaging system. More specifically, the invention involves apparatus and methods for determining the total energy of a continuously under-sampled energy signal utilizing the measured event arrival time.
2. Description of the Related Art
In a positron emission tomograph (PET) imaging system, a patient is injected with a radioactively tagged substance that the body normally metabolizes in some fashion. The radioactive tag used is a positron-emitting isotope of either an element found in the substance or an element that is substituted for another element in the substance. For example, a widely used isotope is the positron-emitting isotope of fluorine, 18F. This isotope is substituted, through a chemical synthesis process, for hydrogen in complex compounds such as glucose-forming fluro-deoxyglucose (FDG). When FDG is injected into a patient, the body will attempt to use it in the same fashion as it would normal glucose. Thus, there will be higher concentrations of positron emitters in areas where glucose is metabolized at higher levels, such as the brain, muscle tissue (the heart), and tumors.
As the FDG or other radiopharmaceutical isotopes decay in the body, they discharge positively charged particles called positrons. Upon discharge, the positrons encounter electrons, and both are annihilated. As a result of each annihilation event, gamma rays are generated in the form of a pair of diametrically opposed photons approximately 180 degrees (angular) apart. By detecting these annihilation xe2x80x9cevent pairsxe2x80x9d for a period of time, the isotope distribution in a cross section of the body can be reconstructed. These events are mapped within the patient""s body, thus allowing for the quantitative measurement of metabolic, biochemical, and functional activity in living tissue. More specifically, PET images (often in conjunction with an assumed physiologic model) are used to evaluate a variety of physiologic parameters such as glucose metabolic rate, cerebral blood flow, tissue viability, oxygen metabolism, and in vivo brain neuron activity.
Mechanically, a PET scanner consists of a bed or gurney and a gantry, which is typically mounted inside an enclosure with a tunnel through the center, through which the bed traverses. The patient, who has been treated with a radiopharmaceutical, lies on the bed, which is then inserted into the tunnel formed by the gantry. Traditionally, PET scanners are comprised of one or more fixed rings of detectors, surrounding the patient on all sides. Some newer scanners use a partial ring of detectors and the ring revolves around the tunnel. The gantry contains the detectors and a portion of the processing equipment. Signals from the gantry are ultimately fed into a computer system where the data is then processed to produce images. Detectors on the detector rings encircling the patient detect the gamma rays, one on either side of the patient. The processing electronics determine when in time each gamma ray occurs. Therefore, when two detectors on opposite sides of the patient have detected gamma rays that occurred within some time window of each other, it is safe to assume that the positron-electron interaction occurred somewhere along the line connecting the two detectors.
The scanner detectors use a scintillator to detect the gamma rays. Suitable material used for the scintillator includes, but is not limited to, either lutetium oxyorthosilicate (LSO) or bismuth germanate (BGO). The output from the scintillator is in the form of light pulses corresponding to the interactions of gamma rays within the crystal. A photodetector, typically a photomultiplier tube (PMT) or an avalanche photodiode, detects the light pulses and converts them into electrical signals, which are filtered and sent to a processing system.
To accurately measure the energy absorbed from a gamma ray interacting in the detector, the total light from a crystal scintillation event must be determined by integrating the signal (light detected by the PMT). This integration is traditionally performed using analog circuitry via a gated integrator or using the summation of digital samples of the signal. However, in order to get a good estimate of the energy using digital integration, one must acquire a sufficient number of samples of the energy signal. The energy estimate degrades as the number of samples decreases. The practical sampling rate is limited by commercially available analog-to-digital converters (ADC). This sampling limit is typically not a problem for energy signals of long duration. However, for short duration scintillation signals, the sampling frequency may limit the number of samples to as few as four or five samples.
An apparatus and method for determining the total energy of a continuously under-sampled energy signal resulting from an annihilation event is provided. A gamma ray from an annihilation event interacts with a scintillator crystal, such as lutetium oxyorthosilicate (LSO), which produces a light output sensed by a photomultiplier tube (PMT). The PMT output signal is sensed by a constant fraction discriminator (CFD) followed by a time-to-digital converter (TDC), precisely registering the time of occurrence of the light pulse. The PMT output signal is shaped with a low-pass filter having an approximate 25 ns shaping time used as an anti-aliasing filter, followed by an analog-to-digital converter (ADC). The sample time for the ADC is such that only 3 or 4 samples of the shaped signal are made. The time relationship of the ADC samples to the start of the signal is known due to the known synchronous relationship between the TDC clock and the ADC sample clock. Because the shape of the sampled filtered energy signal is known and by matching the samples to the shape using the TDC time information, a corrected estimate of the actual gamma ray energy can be calculated. The corrected energy of the shaped signal is calculated from the time relationship by calculating a new amplitude for each sample.